The present invention relates generally to amplifier circuitry for driving speakers and other loads.
Basically, amplifier circuitry (power amplifiers) for driving speakers and other loads produce voltage outputs and supply voltage signals to each load, at which time an electric current value is determined by impedance of the load. In the case where the load is a speaker, the impedance depends not only on the model of the speaker but also on a frequency employed. Therefore, the power amplifier has to adjust to a considerably great range of load impedance.
The output power from the power amplifier equals a product between the output voltage and the output current. The maximum output voltage depends on a power-supply voltage; however, because the power supply circuit has a regulation characteristic, the power-supply voltage decreases as the load current (output current) increases. Therefore, as load resistance decreases, the load current increases, but the maximum output voltage decreases. Consequently, although the maximum output power does not completely relate inversely to the load resistance, it generally changes in an increasing direction, so that the amplifier would break down due to excess current or excess power (heat), unless some means for limiting the output signal is provided somewhere.
Thus, most power amplifiers are provided with some form of limiter circuit (protection circuit) for protective purposes.
Among various known examples of the limiter circuit are (1) a voltage limiter for limiting the output voltage, (2) a current limiter for limiting the output current, a PC limiter for limiting the power consumed by the amplifier, etc.
(1) Voltage Limiter:
In the case of amplifiers, clipping can be effected with a power-supply voltage in the absence of a limiter circuit, and thus no significant inconvenience would be encountered even where no voltage limiter circuit is provided; as a matter of fact, many amplifiers are provided with no limiter circuit. In such cases, because a clipped voltage of an output depends on the power-supply voltage, the clipped voltage varies with variation in the power-supply voltage. Also, in general, the power-supply voltage decreases as a load current increases as note earlier, so that the clipped voltage decreases too. This may appear to be disadvantageous from the viewpoint of an “ideal amplifier”, but, in reality, it is not so. Ordinary constant-voltage limiter tends to present a great difference in output between loads of 8. and 2. (it may approach a value of an ideal amplifier and ideal power supply only seemingly); therefore, the constant-voltage limiter achieves little practical advantageous result, except in cases where such an effect is pursued on purpose.
(2) Current Limiter:
The current limiter performs a function essential for protection of an output element at the time of a load short-circuit, and it is activated when an output current exceeding a limit value flows when a load is short-circuited or has very low impedance. However, the current limiter would present the inconveniences that current clipping results in distorted signal waveforms because the impedance of a speaker includes a reactance component and that the output is a limited current that may prevent proper damping of the speaker. These inconveniences would lead to generation of unusual sounds uncomfortable to the auditory sense, and thus it is not desirable to activate the current limiter frequently for power control purposes during music reproduction. Therefore, the current limiter is normally used for the purpose of protecting and preventing the amplifier from breaking down due to an unusual load condition.
(3) PC Limiter:
The PC limiter is arranged to impart a limited current value of a current limiter with output-voltage depending characteristics. The PC limiter is advantageous in that it can set a small current limiting value and thus reduce physical burdens on an output element at the time of a load short-circuit. The PC limiter is more effective than the current limiter in terms of protection of the output element; however, because of its current limiting function, unusual sounds would be easily noticeable if the PC limiter is activated during music reproduction, as in the case of the above-mentioned current limiter.
In Japanese Patent Application Laid-open Publication No. HEI-10-173446, there has been proposed a power amplifier of the type which, in order to prevent an excessive output, compares an output voltage of a power amplifier section to a predetermined reference voltage value so as to lower an input signal level when the output voltage has exceeded the reference voltage, and in which the predetermined reference voltage is set to any one of a plurality of voltage values in accordance with a current supplied to the power amplifier section depending on a load.
Where an ideal amplifier, supplied with a current from an ideal power supply (complete constant-voltage power supply), provides a maximum output of 100 W for an 8 . load, the ideal amplifier will provide a maximum output of 200 W for an 4 . load and maximum output of 400 W for an 2 . load. There exist amplifiers designed simply on the basis of the though that amplifiers having maximum output characteristics close to an ideal amplifier are good amplifiers; however, it is practically necessary for audio amplifiers to adjust to loads of up to about 2 ., in which case the amplifiers have to be designed to stand an output of 400 W and therefore tend to increase in size. Further, because such amplifiers can produce an output of only 100 W when driving an 8 . speaker that is most popular on the market, they can never be preferable amplifiers from the viewpoints of rationality and practicality.
Namely, it is desirable that the maximum output of amplifiers be prevented from varying greatly even when the load impedance varies greatly, for example, from 2 . to 8 .